Why does my render quality decrease when I open my file in an older version, then bring it back to the current version?
Check the "multiple importance sampling" option in the world panel. Sometime recently, I believe post-2.76, it was changed to be enabled by default. Disabling it would produce the result you see here, and might happen if you change Blender versions without explicitly setting that flag.

See this answer for what that setting does: https://blender.stackexchange.com/a/8156/8081